1. Field of the Invention
The present invention relates to a controller for controlling a secondary battery used as a power source of an electronic apparatus, which includes a secondary battery such as a lithium ion battery as an operating power source, a method for controlling a secondary battery during operation, and an electronic apparatus.
2. Description of Related Art
In recent years, electronic devices have become increasingly smaller and lighter, promoting widespread use of portable equipment. With the widespread use of the portable equipment (also called mobile equipment), there is a rapidly growing demand for a rechargeable secondary battery that is to be used as a power source for the portable equipment.
In order to ensure long time operation of the mobile equipment, attempts have been made to increase the battery capacity of a secondary battery. However, the battery capacity of a secondary battery is limited. Moreover, a secondary battery that is sufficient by itself to provide long time operation of the mobile equipment that requires a relatively large current (e.g., a notebook computer) is still under development. Compared to the use of one large and heavy secondary battery, the use of two or more secondary batteries, each having a predetermined size, as needed is advantageous to users because they can choose the number of secondary batteries in accordance with the expected operating conditions and operating time. For the production of the secondary battery, this is also advantageous to manufacturers in terms of design and manufacturing cost, since many secondary batteries having a predetermined size can be produced.
On the other hand, when single mobile equipment uses a plurality of secondary batteries, the secondary batteries need to be controlled as follows. First, the number of the secondary batteries actually included in the mobile equipment is confirmed. Then, if there are two or more available secondary batteries, the conditions such as the amount of current remaining in each of the available secondary batteries are determined in selecting the secondary battery to be used. If one of the secondary batteries is found to be not available, it is switched smoothly to another secondary battery.
As described above, in the case of using two or more secondary batteries, the following battery control method has been proposed (see Patent Document 1: JP 2005-323483 A). In this method, to reduce the burden of controlling the secondary batteries on the electronic apparatus, the individual secondary batteries have both a communication function and a control function, so that the connection states of the secondary batteries and the timing of switching between the secondary batteries can be transmitted, recorded, and controlled.
FIG. 8 is a block diagram of a conventional electronic apparatus using a plurality of secondary batteries, as disclosed in Patent Document 1.
The conventional electronic apparatus 102 includes a first battery pack 103 and a second battery pack 104 that are connected to a main power source 101. A load 105 is a driving circuit of the electronic apparatus 102, and is operated by switching the electric power between the external main power source 101, the first battery pack 103, and the second battery pack 104.
The first battery pack 103 and the second battery pack 104 have the same configuration and are removable from the electronic apparatus 102. The first battery pack 103 and the second battery pack 104 contain chargeable and dischargeable power sources (not shown in FIG. 8) and control circuits 111, 121 provided with state setting portions 112, 122 that are set according to the states of the battery packs, respectively.
The state setting portions 112, 122 are set according to the states of the first and second battery packs 103, 104, e.g., according to whether the first and second battery packs 103, 104 are attached to the electronic apparatus 102, or whether the first and second battery packs 103, 104 are available because the charged electric power still remains.
When the first battery pack 103 is attached to the electronic apparatus 102 in FIG. 8, an attachment detecting terminal of the first battery pack 103 detects the attachment of the first battery pack 103. Moreover, after confirming that another battery pack has not been attached yet, the state setting portion 112 of the first battery pack 103 is set to an operating state. The control circuit 111 of the first battery pack 103 outputs a presence indication signal through a presence indication line 131. This makes it possible to recognize that the first battery pack 103 has been attached and is being operated at the time another battery pack (i.e., the second battery pack 104) is connected to the electronic apparatus 102. Then, the first battery pack 103 supplies electric power to the load 105.
Subsequently, when the second battery pack 104 is attached to the electronic apparatus 102, the state setting portion 122 of the second battery pack 104 is set to a non-operating state based on the presence indication signal from the first battery pack 103. Thus, the control circuit 121 of the second battery pack 104 does not allow the second battery pack 104 to supply electric power.
Thereafter, when the first battery pack 103 is discharged and becomes unavailable, the control circuit 111 of the first battery pack 103 erases the presence indication signal in the state setting portion 112. The control circuit 121 of the second battery pack 104 detects that the presence indication signal of the first battery pack 103 is not input from the presence indication line 131, and the state setting portion 122 of the second battery pack 104 is set to an operating state. Consequently, the second battery pack 104 starts supplying electric power.
As described above, the first and second battery packs 103, 104 detect the presence indication signal from each other to know whether another battery pack has been attached and whether another battery pack is being operated. In this manner, the electric power can be supplied continuously from two or more battery packs. In the electronic apparatus 102 shown in FIG. 8, the charge of the battery packs also is controlled by the control circuits 111, 121 contained in the first and second battery packs 103, 104. Moreover, in the electronic apparatus 102, the states of the first and second battery packs 103, 104 are transmitted through a state information communication line 132 to a CPU 106 of the main body and then monitored.
The above method for controlling a plurality of secondary batteries in the conventional electronic apparatus uses the control circuits of the respective battery packs to control the charge and discharge of the battery packs, thereby achieving a continuous electric power supply in accordance with the attachment of the individual battery packs and an efficient charging process while the battery packs are connected to the external power source.
However, in the conventional electronic apparatus, when the electric power is supplied from the secondary batteries, the battery packs are used in the order they are attached to the electronic apparatus. Accordingly, one of the battery packs supplies electric power continuously until it is not available, and then this battery pack is switched to the other battery pack. Thus, the battery temperature is likely to rise due to the continuous use and may exceed the temperature limit depending on the operating conditions, so that the time at which predetermined electric power cannot be supplied occurs earlier. Moreover, in the conventional electronic apparatus, once the secondary battery is considered to have finished supplying electric power, it is not expected that the secondary battery will be reused. Therefore, even if the secondary battery is only subjected to a temporary reduction in output, e.g., because of the ambient temperature, and then is restored to the available state, the electric power cannot be supplied from this available secondary battery again. Thus, the total operating time of the secondary battery cannot be increased sufficiently.